DISTANCE, SIZE, AND MASS. 7

altogether inadequate to the requirements of that im-mensely difficult problem. * But he does not appear tohave investigated the subject of the moon’s distance.Somewhat more than a century and a quarter later,Hipparchus attacked both problems ; the first withno better success than had rewarded Aristarchus , butthe second by a method which was probably verysuccessful in his hands, though it is from his successorPtolemy that we learn the actual results of observa-tions applied according to the ideas of Hipparchus .

It would appear that the scrutiny of the moon’smotions,—with the object of determining her pathamong the stars, and the exact laws according towhich she traverses that path,—led Hipparchus toattack the problem of determining the moon’s distance.We know that his observations were so carefully pur-sued that he determined the eccentricity of the moon’spath, and its inclination to the sun’s annual path onthe star-vault. It is also highly probable that hedetected a certain peculiarity of the moon’s motion,called the evection, which will bo described further on.Whether this is so, or whether the discovery shouldbe ascribed to Ptolemy , it is certain that the laboursof Hipparchus could not have led to the resultsactually obtained, without his having noticed certaineffects due to the relative nearness of the moon ascompared with the other celestial bodies. The studyof these effects probably enabled him to form a fairestimate of the moon’s distance.

* His method is described in my treatise on the Sun (p. 7).